1. Field of the Invention
The present invention relates to light emitting diode (hereinafter referred to as “LED”) devices and methods for fabricating the same, and more particularly, to AlGaN-based multi-quantum well ultraviolet (UV) LED devices with improved light emission intensity and methods for fabricating the same.
2. Background and Relevant Art
Group III-V compound semiconductor materials which are representative of the third generation semiconductor materials have many excellent properties, and particularly in terms of optical application, alloys {Ga(Al, In)N} consisting of Ga, Al, In and N may be suitable for both the visible light region and the near-UV light region. Furthermore, group III nitrides having wurtzite structure are of direct band gap and very suitable for photoelectronic device application. AlGaN-based multi-quantum well UV LED devices, as a focus of current UV photoelectronic device development, have exhibited great advantages especially for UV light region. However, as the wavelength of light emitted from the LED devices becomes shorter, Al composition in active layer of the GaN-based LED devices is increasingly high. Therefore, it is difficult to fabricate a high quality AlGaN material. Particularly, the external quantum efficiency and the light power of the UV LED devices are very low because the AlGaN material absorbs UV light strongly, which have become a bottleneck for developing the UV LED devices and a problem to be solved currently.
AlGaN-based multi-quantum well UV LED devices have a bright application prospect. Firstly, there has been a breakthrough in the development of GaN-based blue-green light LED devices, the GaN-based blue-green light LED devices have been commercially available and applied in view illumination, large-size backlight source, light communication, etc. Secondly, white solid illumination LED devices have been developing rapidly, initiating the 3rd illumination revolution. Thirdly, as well developed in the visible light field, short wavelength UV light have been received more and more attention, the UV light has been found to be valuable in screen printing, polymer solidification, environment protection, white light illumination and military survey.
Presently, some new material growth methods or new structures are used for reducing damage due to stress to the quality of AlGaN material, in order to improve the growth quality of AlGaN material, thereby improving light emission performance of the UV LED devices. These methods comprising:
A UV LED device with a wavelength less than 300 nm was first achieved in South Carolina State University in 2002. An LED device with a wavelength of 285 nm was fabricated on a sapphire substrate, and for a 200μ×200 μchip, the power was 0.15 mW at a pulse current of 400 mA, the maximum power of which is up to 0.25 mW after an improvement for p-type and n-type contact resistances, referring to V. Adivarahan, J. P. Zhang, A. Chitnis, et al, “sub-Milliwatt Power III-N Light Emitting Diodes at 285 nm,” Jpn. J. Appl. Phy, 2002, 41: L435. Subsequently, LED devices with emission wavelengths of 280 nm, 269 nm, 265 nm were achieved in sequence, the maximum power of which exceeds 1 mW, referring to W. H. Sun, J. P. Zhang, V. Adivarahan, et al. “AlGaN-based 280 nm light-emitting diodes with continuous wave powers in excess of 1.5 mW” Appl Phys Lett, 2004, 85 (4): 531; V. Adivarahan, S. Wu, J. P. Zhang, et al. “High-efficiency 269 nm emission deep ultraviolet light-emitting diodes” Appl. Phys. Lett, 2004, 84 (23):4762; Y. Bilenko, A. Lunev, X. Hu, et al. “10 Milliwatt Pulse Operation of 265 nm AlGaN Light Emitting Diodes” Jpn. J. Appl. Phys, 2005, 44:L98. In order to improve current transmission and to reduce heat effect, for a small area chip of 100 μm×100 μm, connected in a 2×2 array with a flip-chip structure, the maximum power of 24 mW was obtained at a wavelength of 280 nm and the maximum external quantum efficiency is up to 0.35%, referring to W. H. Sun, J. P. Zhang, V. Adivarahan, et al. “AlGaN-based 280 nm light-emitting diodes with continuous wave powers in excess of 1.5 mW” Appl. Phys. Lett, 2004, 85 (4): 531. An LED device with a wavelength of 250 nm was further fabricated in 2004, and for a 200μ×200 μchip, the maximum power is nearly 0.6 mW; however, the external quantum efficiency is only 0.01%, referring to V. Adivarahan, W. H. Sun, A. Chitnis, et al. “250 nm AlGaN light-emitting diodes” Appl. Phys. Lett, 2004, 85 (12): 2175.
A further development for deep UV, particularly in the wavelength range of 280-290 nm, was achieved in Northwest University and Kansas State University in 2004, referring to Fischer. A. J, Allerman. A. A, et al. “Room-temperature direct current operation of 290 nm Light-emitting diodes with milliwatt power level” [J]. Appl. Phys. Lett, 2004, 84 (20):3394. A high power UV LED device of 1 mm×1 mm size was obtained using interdigitated contact geometry for improving internal current spreading and a flip-chip structure for improving heat dissipation, with an emission wavelength of 290 nm, an emission efficiency of 1.34 mW at a direct current of 300 mA, and the external quantum efficiency of 0.11%, referring to Kim. K. H, Fan. Z. Y, Khizar M, et al. “AlGaN-based ultraviolet light-emitting diodes grown on AlN epilayers” [J]. Appl. Phys. Lett, 2004, 85 (20):4777.
Also in 2004, deep UV LED devices with wavelengths of 250 nm and 255 nm were further developed in South Carolina State University. An AlGaN/AlN super-lattice structure was used in a bottom buffer layer, and an AlGaN barrier layer with high quality was grown, and thereby a deep UV LED device of 200 μm×200 μm size was fabricated, the emission efficiencies of which are 0.16 mW and 0.57 mW, respectively at pulse currents of 300 mA and 400 mA. However, due to the bottom emission type, the emission efficiency was still low, referring to V. Adivarahan, W. H. Sun, A. Chitnis, M. Shatalov, S. Wu, H. P. Maruska, M. Asif. Khan. “250 nm AlGaN light-emitting diodes” Appl. Phys. Lett, 2004, 85 (12):2175.
Deep UV LED devices with wavelengths in the range of 231-261 nm were further developed in Saitama University in 2007 in Japan. Since an AlN buffer layer was formed through pulse grown technique and dislocation defect density of AlN layer was further reduced, an AlGaN layer with high Al composition was grown, thereby the light power and external quantum efficiency of the deep UV LED device with a wavelength of 261 nm were 1.65 nW and 0.23%, respectively, referring to Hirayama Hideki, Yatabe Tohru, Noguchi Norimichi, Ohashi Tomoaki, Kamata Norihiko “231-261 nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN multilayer buffers grown by ammonia pulse-flow method on sapphire” Appl. Phys. Lett, 2007, 91 (7):071901-1.
In summary, currently, AlGaN-based deep UV LED devices are mainly fabricated as the bottom emission type, and the top emission type LED devices haven't been studied in depth. With decrease of the emission wavelength, the bottom buffer layer absorbs UV light increasingly, thereby producing an adverse effect on the light emission power and external quantum efficiency. Currently, the quality of epitaxial layers is improved mainly by modifying the structure of devices. However, existing bottom emission type LED devices have following disadvantages: first, the emission path of light is too long and light loss on the path is too large, causing the external quantum efficiency to be low. Secondly, the crystallizing quality of bottom AlN buffer layer is poor, thereby causing increased non-radiative recombination centers in the material and increased UV light being absorbed. Thirdly, there are increased defects of trapping photons in the bottom buffer layer under electrical stress, adversely affecting the reliability of the devices.